Message size of >=520 bytes result in inconsistent publish timing

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Publishing a message of size 519 bytes at 100Hz results in this output for 'rostopic hz':

average rate: 99.997 min: 0.010s max: 0.010s std dev: 0.00006s window: 996

Increasing the message size by one byte to 520 bytes results in this output for 'rostopic hz':

average rate: 99.985 min: 0.000s max: 0.044s std dev: 0.01651s window: 4908

This is a custom message defined as an array of int8. But the behavior is independent of message type. Any message of size >519bytes exhibits the same effect. The behavior is also independent of Python/C++ and queue size. The effect is dependent on publish rate, but starts to appear around a rate of 35Hz.

The much larger std. dev. is caused by a seeming buffer effect whereby the larger messages come in groups of 3-5 messages vs. the much more consistently timed publishing rate of the smaller message.

Is this a known behavior? It is surprising to me because 520 bytes isn't a terribly large message size. The nav_msgs::Odometry message is larger.

While I used a custom message to narrow down the precise behavior above, this code that simply uses the Odometry message (>519 bytes) will replicate the issue:

#!/usr/bin/env python

import rospy
from nav_msgs.msg import Odometry


def odomer():
    pub=rospy.Publisher('odomer',Odometry,queue_size=100)
    rospy.init_node('odomer',anonymous=True)
    rate=rospy.Rate(40)
    while not rospy.is_shutdown():
        odom_msg = Odometry()
        pub.publish(odom_msg)
        rate.sleep()


if __name__ == '__main__':
    try:
        odomer()
    except rospy.ROSInterruptException:
        pass